1. Field of the Invention
The present invention relates to an image pickup device suitable for a single-plate image pickup device for use in, e.g., a recorder integrated with a camera or a color video camera.
2. Related Background Art
As is well known, a conventional single-plate CCD camera is constituted as shown in FIG. 13. The operation of this single-plate CCD camera will be described below with reference to FIG. 13.
Light from an object to be imaged, which is incident through an optical system 201 such as a lens, is converted into an electrical signal by a solid-state sensor 202 such as a CCD. The electrical signal is then sampled and converted into digital data by an A/D converter 203. The CCD 202 is a complementary color type single-plate CCD shown in FIG. 14A, in which color difference components and a luminance component are frequency-multiplexed as shown in FIG. 14B.
In the single-plate CCD camera with the above arrangement, color difference signals are extracted by a procedure to be described below. That is, a color separation circuit 204 extracts color modulation components CR and CB and a luminance component Y shown in FIG. 14B from the digital CCD output. A matrix circuit 205 converts these components into signals of three primary colors R, G, and B.
These R, G, and B signals are applied to multipliers 206a, 206b, and 206c and multiplied by the values of coefficients 208a, 208b, and 208c of registers, respectively, thereby performing adjustment of white balance. The resulting signals are then subjected to .gamma. correction performed by gamma circuits 207a to 207c and converted into a Y signal by a matrix circuit 209 which is represented by the following equation: EQU Y=0.59R+0.3G+0.11B (1)
Subsequently, subtracters 210 and 211 are used to calculate the difference between the R and Y signals and the difference between the B and Y signals, respectively, thereby obtaining signals R-Y and B-Y as color difference signals.
Thereafter, the R-Y and B-Y signals are subjected to color correction performed by a linear matrix circuit 212 for performing color correction, and quadrature-modulated into digital chroma signals by a modulator 215 after being passed through faders 213 and 214, respectively, which are controlled by a controller (not shown). These digital signals are converted into analog signals by a D/A converter 216 and output from an output terminal 217 as an analog chroma signal C.
On the other hand, the luminance signal Y is, in order to keep its band (resolution), passed through a low-pass filter (LPF) 218 for suppressing modulation components of color differences, thereby correcting characteristics which are damped by the lens optical system 201 or the like. The resulting signal is then passed through an aperture correction unit 219 to give sharpness to an image.
Thereafter, the signal is subjected to gamma correction by passing through a gamma correction unit 220 and converted into an analog signal through a fader 222 and a D/A converter 223 like in the color difference processing. The consequent analog signal is output from an output terminal 224.
When a color bar chart is photographed, the luminance signal Y which is obtained as described above is different from an output obtained by a three-plate camera. FIG. 15A shows an output from a three-plate camera, and FIG. 15B shows the luminance signal Y of the single-plate camera described above. This phenomenon will be described below.
As shown in FIG. 17, a luminance output Y.sub.3 of a three-plate camera is obtained by performing gamma correction for R, G, and B signals, multiplying the resulting R, G, and B signals by coefficients of 0.59, 0.3, and 0.11, respectively, and adding these signals.
A luminance output Y.sub.1 of a single-plate camera is, on the other hand, obtained by passing a CCD output through a low-pass filter and performing gamma correction for the resulting output. R, G, and B components of this CCD output are, for example, R:G:B=2:3:2, and these components are subjected to gamma correction. Therefore, the output as shown in FIG. 15B is obtained. This signal has larger errors than those of the luminance output Y.sub.3 of the three-plate camera and hence has a drawback of a poor color reproducibility in objects to be photographed in dark colors.